The invention relates to a mandrel for the coaxial external machining of workpieces with internal toothing, consisting of a reception sleeve with an external toothing corresponding to the workpiece toothing, and also a clamping member that engages with the workpiece toothing and that can be rotated with respect to the reception sleeve by means of a rod arranged within said reception sleeve, by means of which clamping member the internal toothing of the workpiece can be brought to bear in a circumferential direction against the external toothing of the reception sleeve.
Such mandrels are used for a whole range of applications. A particularly important field of application is the manufacture of gears and coupling parts whose boss has an internal toothing by means of which they can be rigidly mounted on a shaft having the corresponding external toothing. For example, in the case of gears the pitch circle of the gear rim must be arranged as concentrically as possible with respect to the axis of the inner toothing, since otherwise drive systems in which such gears are used will produce considerable running noises. The coaxial position of the pitch circle of the gear rim with respect to the axis of the internal toothing assumes that the gear is completely centered while being machined by milling, planing and grinding, etc. The centric clamping of the gear is particularly important if, during one of the first work stages, a cylindrical twisting force is produced on the outer surface of the gear that is intended to clamp the workpiece during its further machining. Any errors in alignment would of course have an additive effect in the most unfavorable case, with the result that the deleterious effect on a finished gear would be magnified even further. A whole series of mandrels are known for clamping workpieces as centrically as possible, but these do not satisfy all the postulated requirements. With regard to the shape and contour of the teeth profile, the DIN Standards 5480 and 5482 have to be observed in most applications.
While they are being machined, the workpieces provided with internal toothing are to be held on the mandrel under conditions, insofar as possible, which will be identical with their final mounting on a complementary shaft provided with external toothing. The unachievable ideal would be a one-piece, massive, seamless mandrel without any cross-sectional reduction or the like.
The invention is therefore directed to the primary object of creating a mandrel on which the workpiece will be held during machining just about as securely as it will be after its final installation. It is against this background that the following is set forth regarding the state of the art.
U.S. Pat. No. 2,806,705 does not disclose a mandrel whose reception sleeve is continuous over the entire clamping length. Instead, in the area of the clamping length the mandrel consists of a total of three parts which are disposed in axial succession and are rotatable relative to one another, and it is intended for the simultaneous clamping of a plurality of workpieces. Since the workpieces rotate a relative to one another, the external toothing of the two workpieces has a different angular position with respect to the internal toothing.
The radial divisions between the parts result in a considerable weakening of the moment of resistance, so that the workpieces can be deflected resiliently under the machining forces. Deflection can be countered only partially by means of an extremely slight radial clearance between the relatively movable parts. In this case, however, another disadvantage manifests itself, the cause of which lies in unavoidable inaccuracies in the toothing both on the mandrel and on the workpiece. A clamping member having a plurality of teeth accordingly first comes into contact with the workpiece with only one tooth. On account of the curved shape of the toothing (involute engagement angle), this results in a radial component of force at the first point of contact. Since this component of force is initially unopposed by any reaction force, the workpiece is positively forced away from the (assumed) concentric position until additional, i.e., thinner, teeth on the workpiece make contact.
U.S. Pat. No. 2,805,864 discloses a mandrel having within its clamping length two parts and a division between them. The first part is made in the form of a ring with external toothing, and it is held on the other part by means of three screws distributed on the circumference. The ring has to be made removable because otherwise the shaft could not be assembled with the clamping member.
There is virtually no way of compensating for the considerable weakening of the moment of resistance caused by the division (the cross section is interrupted precisely in the outer circumference). The shaft is of virtually no help in stiffening. Radial displacement of the ring is prevented by the external prolongation of the shaft. To enable the ring to be supported on the prolongation there must be a very close fit both between the ring and the prolongation and between the shaft and the bore. This close fit, however, simultaneously prevents the shaft and the clamping member from being deflected under any radial forces, so that excentric radial forces between workpiece and clamping member again must result in an excentric displacement of the workpiece on the mandrel.
It is to be considered as an additional disadvantage of the known mandrels that the many parts required have to be made with the needed precision, so that the cost and difficulty of design and manufacture are correspondingly great. For small workpiece dimensions (pitch circle diameters less than 25 mm) the known methods of construction are unfeasible for practical reasons.